1. Field of the Invention
The present invention relates to a functional substrate and an electronic circuit substrate using it.
2. Description of the Prior Art
Heretofore, an insulator substrate on which conductive circuits and resistors are mounted has been mainly employed as a substrate for an electronic circuit using ceramics, and electronic parts such as capacitor and the like have been mainly used as a chip part mounted on an insulator substrate. An example of such a substrate for an electronic circuit is illustrated in FIG. 1, wherein 1 is a ceramic insulator substrate, 2 a conductive circuit, 3 a resistor, 4 an insulator layer for a cover, and 5 a chip capacitor.
However, when chip parts such as a capacitor and the like are mounted on the outside of a substrate as in FIG. 1, the substrate becomes bulky, and guaranteeing a mounting site for the chip part inevitably restricts design of circuits. As a result, the recent requirement for miniaturization of electronic circuits is not sufficiently satisfied.
On the other hand, there has been developed the following technique for fabricating capacitors as electronic parts using ceramics: A material of the barium titanate series sintered ceramics is subjected to a sintering operation together with impurities to render them semiconductive followed by a firing using metal or metal oxide diffusion source to give dielectrics and then a solid capacitor is produced. However, while such capacitors, can be used as chips, but there has not yet been established a technique which permits a plurality of capacitors to be built-in in a circuit substrate, and therefore, the capacitor does not sufficiently contribute to miniaturizing circuit substrates.
Recently, as the technique concerning a laminated ceramic capacitor is extended, one of the abovementioned capacitor parts has the form of a substrate having a built-in capacitor therein and a conductive circuit and a resistor on the surface. In that type of substrate, however, the characteristic range of a built-in capacitor is restricted by dielectrics constituting the substrate. When the circuit constant is within the restriction range, the substrate is very effective for miniaturizing an electronic circuit.
But, there are serious defects concerning the substrate.
First, one of the defects is high cost. In other words, procedure for forming the capacitor comprises laminating ceramics, and production facilities becomes complex, and hence many steps are required. It is, also, required to use a lot of relatively expensive electrode materials.
Second, there is a defect concerning the construction of the circuit.
To form plural capacitor-forming units in the same substrate, referring circuit constant, the units should be constructed by combining plural dielectrics depending on the circuit constant and laminating. Therefore, there exist defects in miniaturizing a circuit and in the degree of freedom available in designing a circuit, and hence fields of circuit the fields to which the circuit can feasibly be applied are limited.
Third, it is necessary for a laminated ceramic capacitor to have two through-holes per capacitor, and hence this type of substrate having built-in capacitors has a lot of through-holes. Consequently, the substrate has, also, defect that the surface area increases with through-holes.
FIG. 2 is an example of the type of ceramic substrate, wherein 1-3 have the same meaning, respectively as described in FIGS. 1, 6 and 7 are constitution units of built-in laminated capacitors in a substrate, 8 is a chip part mounted on a substrate, 9 is a through-hole.
In view of the foregoing, in the case of improving laminated ceramic capacitors and forming plural built-in capacitors in a substrate, there are serious limitations not only concerning cost but also concerning miniaturization of the circuit and degree of freedom for designing circuits.